1. Technical Field
This invention is related to the field of surface acoustic wave ("SAW") filters. In particular, the invention is related to SAW filters that are useful in UHF radio transceivers, either fixed or mobile, that require frequency selectivity and image rejection in the transceiver front-end prior to a differential mixer. However, the teachings of this invention are applicable to any type of electronic circuit that employs a SAW filter having balanced outputs or inputs as the case may be.
2. Description of the Related Art
Dual-mode leaky SAW structures were reported in the literature by Morita in 1992 (T. Morita, Y. Watanabe, M. Tanaka and Y. Nakazawa, "Wideband Low Loss Double Mode SAW filters," Proc. 1992 IEEE Ultrasonics Symp., October 20-23) The layout of this type of SAW consists of a central inter-digital transducer ("IDT") surrounded by two other IDTs connected in parallel with a external connection. On either end of the IDTs are reflectors. This resonator structure is usually then cascaded with another identical device to produce a filter with reasonably good attributes. Usual operation of this filter is to have a single input and a single output. In other words, these SAW devices do not provide a differential output signal.
Recently, the trend has been to design the front-end of a radio transceiver with a differential input to the first mixer. The advantage of this type of design is to take advantage of the common mode rejection between the two differential inputs. This method then requires a differential SAW filter before the first mixer. Such a differential SAW filter is shown in FIG. 1, described in more detail below. As with most such differential SAWs, the structure shown in FIG. 1 suffers from an imbalance of the differential outputs. FIGS. 2 and 3 depict the imbalance in the differential outputs in this type of SAW. This imbalance is due to the nonsymmetrical coupling within the IDTs of the output stage.
Therefore, there remains a general need in this field for a differential SAW filter having a pair of balanced outputs.